LT3508
APPLICATIONS INFORMATION
is much lower than the switching frequency. A phase-lead
used to prevent excessive discharge of battery-operated
systems.
capacitor (C ) across the feedback divider may improve
PL
the transient response.
If an adjustable UVLO threshold is required, the SHDN
pin can be used. The threshold voltage of the SHDN pin
comparatoris2.63V.Currenthysteresisisaddedabovethe
SHDNthreshold.Thiscanbeusedtosetvoltagehysteresis
of the UVLO using the following:
LT3508
CURRENT MODE
POWER STAGE
V
SW
OUTPUT
ERROR
AMPLIFIER
g
= 2.5S
m
C
PL
R1
FB
–
g
=
m
VH – VL
R3 =
300ꢀS
ESR
+
2M
0.8V
C1
7.5μA
+
GND
V
C
C1
2.63V
R4 =
POLYMER
OR
TANTALUM
CERAMIC
R2
R
C
VH – 2.63V
– 8μA
C
F
C
C
R3
3508 F06
Example:switchingshouldnotstartuntiltheinputisabove
4.75V and is to stop if the input falls below 4V.
Figure 6. Model for Loop Response
VH = 4.75V, VL = 4.0V
Shutdown and Undervoltage Lockout
4.75V – 4V
R3 =
R4 =
= 100k
Figure 7 shows how to add undervoltage lockout (UVLO)
to the LT3508. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
sourcevoltagedrops. Thislookslikeanegativeresistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions.
UVLO prevents the regulator from operating at source
voltages where the problems might occur.
7.5μA
2.63V
4.75V – 2.63V
100k
= 200k
– 8μA
Keep the connection from the resistor to the SHDN pin
short and make sure the interplane or surface capacitance
toswitchingnodesisminimized.Ifhighresistorvaluesare
used,theSHDNpinshouldbebypassedwitha1nFcapacitor
to prevent coupling problems from the switch node.
An internal comparator will force the part into shutdown
below the minimum V
of 3.7V. This feature can be
IN1
Soft-Start
The output of the LT3508 regulates to the lowest voltage
present at either the TRACK/SS pin or an internal 0.8V
reference. A capacitor from the TRACK/SS pin to ground
is charged by an internal 1.2μA current source resulting
in a linear output ramp from 0V to the regulated output
whose duration is given by:
LT3508
V
IN
V
C
–
2.6V
R3
R4
SHDN
+
C1
TRACK/SS
8ꢀA
7.5ꢀA
CSS • 0.8V
tRAMP
=
1.2μA
3508 F07
At power up, internal open-collector ouputs discharge
both TRACK/SS pins. The pins clamp at 1.3V.
Figure 7. Undervoltage Lockout
3508fb
15
Linear [ Linear ]
